1. Field
Implementations of the present invention relate generally to the field of spectral imaging systems and more particularly to the characterization of the spectral signatures of point-like events that evolve rapidly in time relative to the environment in which the events occur. Each of various implementations further involves a method and associated apparatus for approximating the location of a source event within a predetermined field of view based on a set of pre-contrived dispersion patterns correlated with a set of optical dispersion apparatus used in imaging the spectrum associated with the event.
2. Brief Description of an Illustrative Environment and Related Art
Spectral imaging is the art of quantifying the spectral and spatial characteristics of a scene within a “field of view.” Optical devices known generally as imaging spectrometers have been developed for measuring and analyzing the spectral content of electromagnetic radiation in various ranges within the spectrum of optical wavelengths. These include, by way of non-limiting example, the ultraviolet; visible; and near, short-wave, mid-wave and long-wave infrared ranges of the electromagnetic spectrum. For purposes of this specification, and the appended claims, all wavelengths of the electromagnetic spectrum are included within the definition of “light,” regardless of visibility with respect to the human eye. In other words, the terms “light,” “electromagnetic energy” and “electromagnetic radiation” are regarded as wholly interchangeable and may be used interchangeably throughout the specification.
Spectral images are typically acquired by scanning the image of a slit across the image of an overall scene, but many hardware configurations that execute alternative imaging modes are available. A spectral image usually consists of a sequence of monochromatic images, wherein each monochromatic image represents the scene as it would appear when viewed over a limited wavelength band and each image in the sequence is centered at a unique wavelength. Accordingly, spectral images are inherently three-dimensional (i.e., they include two spatial dimensions and one spectral dimension) and, therefore, some type of multiplexing is required in order to acquire and display the data in two dimensions.
Three current and emerging multiplexing methods are (1) temporal multiplexing, (2) multiplexing at the image plane and (3) multiplexing at a pupil. Temporal multiplexing is commonly used to acquire image data; however, temporal multiplexing introduces artifacts when the scene is not static. Therefore, most spectral imagers work well for scenes consisting of static objects, but fail to accurately represent scenes including events that evolve rapidly in time (i.e., dynamic events). Examples of devices that implement temporal multiplexing include sensors that spatially image and temporally scan over wavelength or over the Fourier Transform of frequency, and technologies that record two-dimensional information along one spatial and one spectral dimension and scan along the remaining spatial dimension in time. Among these devices are filter wheel spectrometers, Fourier Transform spectrometers and scanned slit spectrometers.
The most widely implemented spectral imaging technologies involve multiplexing at the image plane. Image-plane multiplexing apparatus are embodied in nearly all consumer digital still cameras and camcorders. In these devices, pigments or dyes are lithographically placed on individual pixels on the focal plane array typically in what is known as a Bayer pattern. Alternative schemes stack pixels on top on one another, or disperse light collected form each pixel.
Multiplexing at an entrance pupil frequently involves an arrangement of multiple parallel cameras wherein in each camera is filtered to a specific spectral band.
When an event location is known, conventional spectral imaging techniques are adequate for characterizing the spectral signature of the event, but when the location is unknown, the performance of conventional sensors is often unsatisfactory. Conventional sensors employ scanning techniques to multiplex the 3-dimensional spectral imagery (i.e., two spatial dimensions and one spectral dimension) onto a 2-dimensional focal plane array. When the event is uncued, neither its location nor its timing are known and the entire field of view must be rapidly scanned to capture the signature. When the sensor is monitoring a large field of view, the integration time at any given pixel is limited and the sensor noise floor swamps the signal.
Accordingly, a need exists for a method and apparatus for analyzing and characterizing the spectral signature of uncued dynamic events in a manner that provides useful information indicative of the spectral temporal evolution of the event and of the event location within a field of view.